1. Field
The present invention relates to a plasma generation reactor. In particular it relates to a reactor that is configured with a plurality of surfaces between two electrically excited electrodes. This structure allows glow discharge operation at high frequencies and high voltage potentials resulting in high overall ionization of target gasses or fluids to be treated.
2. Prior Art
It is known that glow discharge is generated by placing a dielectric material between two electrodes and applying a high-voltage alternating current or a periodic pulse voltage, and in the plasma field resulting there from, active species, radicals, and ions are generated so as to facilitate decomposition and further reaction of a gas or liquid (refer to U.S. Pat. No. 5,414,324). Plasmas formed in this manner are commonly referred to as dielectric barrier discharges. Further it is known that gasses decomposed by the application of dielectric barrier discharge can be reformed into more desirable forms. For example process and automobile exhaust gasses have been treated with such reactors (refer to U.S. Pat. No. 7,507,934).
The plasma and resulting ion formation in dielectric barrier discharges is dependent on the periodic change in the direction of current flow through the reactor. This periodic change in the direction of flow is controlled by the applied AC or pulsed DC voltage and the rate of change of current flow is controlled by the rate of change of the applied voltage, commonly referred to as dV/dt, at the reactor electrodes. Ion formation in the plasma is greatest during the most rapid field change (U.S. Pat. No. 7,399,944). In maximizing ion formation it is generally desirable to operate dielectric barrier discharges at higher frequencies and higher dV/dt.
Ions in dielectric barrier discharges are concentrated at the surfaces inside the plasma field (U.S. Pat. No. 7,298,092). Therefore, increasing surface area in the plasma field is a generally accepted method of increasing reactor plasma generation capacity.
Finally, it is also generally desirable to increase the size of dielectric barrier discharge reactors to accommodate higher volume processing.
In present dielectric barrier discharge reactor designs increasing the reactor size while maintaining a high surface area to gas/fluid volume ratio directly increases the capacitance of the reactor. This higher capacitance is in conflict with the desire to operate at higher frequencies and provide higher dV/dt at the reactor. As a result, present dielectric barrier discharge reactor designs incur practical limitations in achieving increased ion formation rates due to limitations in operating frequency and dV/dt as the surface areas are increased.
One approach to solving the capacitance problem is to increase the space between the plasma generation electrodes. However, wide spaces for plasma generation result in relatively high percentages of gas remaining un-ionized in areas away from surfaces. Further, when recombination of ions into more desirable species is required, a surface to hold the ionized particle until recombination occurs will accelerate the recombination. It has also been shown (U.S. Pat. No. 7,298,092) that these larger distances between electrodes lead to instabilities in plasma current across the dielectric surfaces. These instabilities can cause parts of the reactor to have weak or non-existent plasma current flow. The prior art does not address the current understanding that streamer formation is not only a function of the distance between electrodes, but also a function of the distance between the dielectric surfaces. Finally, streamer formation is exacerbated at the pulse cycle end, or in low frequency driven systems and is the subject of extensive research (U.S. Pat. No. 7,399,944). One method to solve this problem is to simply increase the drive frequency of the reactor. However this is difficult on slower, higher capacitance reactors.
This leads to a desire for a reactor with low capacitance and high surface area to gas/fluid volume ratio indicating that smaller distances between surfaces is desirable. The present invention presents a novel solution to these problems.
Multiple electrode designs as described in U.S. Pat. No. 7,507,934 (see FIG. 1a, 1b, 1c) attempt to solve one of these limitations and describe a method of increasing the reactor surface area to gas ratio. However, these designs involving multiple electrically connected electrodes result in much higher reactor capacitance.
In addition, it is known that multiple electrically interconnected electrode designs such as intermediate electrodes and multiple electrode designs (U.S. Pat. No. 7,507,934) are used in some plasma reactors such as depicted in FIG. 1. These reactor designs rely on multiple electrically driven plasma generation electrodes to generate plasma in the space between them. Along with the limitations noted above, this design has the tendency of the plasma to form filaments and other non-uniform conduction modes, due to the proximity of the relatively low impedance plasma generation electrodes.
As a general class of reactor design, multiple electrode designs suffer from high capacitance and plasma current non-uniformity particularly as the surface area is increased. This characteristic of the prior art has lead to a diversity of electrode designs to mitigate this non-uniform plasma current problem.
Various electrode configurations and designs result in more uniform plasma current conduction than others and further result in more desirable plasma discharges. These include slotted or perforated electrodes such as in U.S. Pat. Nos. 6,005,349 and 6,818,193. In addition perforated dielectrics have been used to enhance the desirable characteristics of plasma as in U.S. Pat. No. 5,872,426. However, using variations of electrodes does not resolve the capacitance issue. In addition these perforated electrodes cause areas of weakened plasma field in areas around the perforations.
As stated above, it has been shown that higher operating frequencies result in higher plasma energy into a reactor. U.S. Pat. No. 7,507,934 discusses frequencies of greater than 100 Hz, but fails to address operation of that design at higher frequencies. Multi-electrode designs such as these increase the capacitance dramatically over two electrode versions making higher frequency operation more problematic and therefore further limiting the efficiency of the reactor.